The present invention relates to a magnetoresistive effect element and a magnetic memory. In particular, the present invention relates to a magnetoresistive effect element and a magnetic memory using a perpendicular magnetization film.
In a magnetic memory such as a magnetic random access memory (MRAM), a magnetoresistive effect element is used as a memory cell. A typical magnetoresistive effect element has a magnetic tunnel junction (MTJ) in which a tunnel barrier layer is interposed between two ferromagnetic layers.
A resistance value of an MTJ varies in accordance with the magnetization state of the two ferromagnetic layers. Specifically, a resistance value (R+ΔR) of an MTJ in the case where the magnetization directions of the two ferromagnetic layers are “not parallel” is larger than a resistance value (R) in the case where they are “parallel”. Consequently, it is possible to vary a resistance value of an MTJ by fixing the magnetization direction of one ferromagnetic layer and inverting the magnetization direction of the other ferromagnetic layer.
Magnitude of such a resistance value is related to a data “1” or “0”. That is, a magnetoresistive effect element memorizes data in a nonvolatile manner by using variation of the resistance value of an MTJ. In the two ferromagnetic layers, a layer with a fixed magnetization direction is hereunder referred to as a “data reference layer”. The other ferromagnetic layer the magnetization state of which varies in accordance with stored data is hereunder referred to as a “data storage layer”.
Data are writted by varying the magnetization state of a data storage layer. As methods for writing data, an external magnetic field application method, a spin injection method, and a domain wall displacement method are named. Patent Literature 1 discloses an MRAM using a spin injection method or a domain wall displacement method. Further, Patent Literature 2 discloses an MRAM of a domain wall displacement type using a perpendicular magnetization film. According to Patent Literature 2, writing current can be reduced sufficiently by forming a data storage layer with a perpendicular magnetization film.
Data are read by making readout current flow between a data storage layer and a data reference layer through a tunnel barrier layer and detecting the magnitude of the resistance value of an MTJ. On this occasion, it is desirable for judging data accurately and quickly that an MR ratio (ΔR/R) is as high as possible. That is, a high MR ratio is indispensable for realizing a good readout property.
Patent literature 3 and Non-Patent Literature 1 disclose a film configuration allowing a high MR ratio to be materialized. According to Patent Literature 3, a tunnel barrier layer is an MgO film of a single-crystal structure and the part of a ferromagnetic layer coming into contact with the tunnel barrier layer is in an amorphous state. According to Non-Patent Literature 1, a tunnel barrier layer is an MgO film and CoFeB films are formed over and under the MgO film as interfaces. That is, a “CoFeB/MgO/CoFeB” structure where an MgO film is interposed between two CoFeB films is formed. It is reported that the “CoFeB/MgO/CoFeB” structure contributes to the improvement of an MR ratio.